Hybrid graphene/organic semiconductor field-effect transistors

Article abstract of DOI:10.1063/1.4737939

In this letter, we report on the improvement of the electronic properties of graphene by capping with π-conjugated organic semiconductor molecules. The off-state current is reduced while the on-state current and mobility are either unaffected or increased. The semiconductors employed included α-sexithiophene and hexadecafluorocopperphthallocyanine. Removal of the organic semiconductor layer results in a return to the original electronic properties. This suggests that weak electronic interactions with organic semiconductors can be a promising approach to favorably alter the transport properties of graphene.

Full text of DOI:10.1063/1.4737939

Hybrid graphene/organic semiconductor field-effect transistors
Tae-Jun Ha, Deji Akinwande, and Ananth Dodabalapur
Citation: Appl. Phys. Lett. 101, 033309 (2012); doi: 10.1063/1.4737939
View online: http://dx.doi.org/10.1063/1.4737939
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APPLIED PHYSICS LETTERS 101, 033309 (2012)
Hybrid graphene/organic semiconductor field-effect transistors
Tae-Jun Ha, Deji Akinwande, and Ananth Dodabalapura)
Microelectronics Research Center, The University of Texas at Austin, Austin, Texas 78758, USA
(Received 15 June 2012; accepted 5 July 2012; published online 19 July 2012)
In this letter, we report on the improvement of the electronic properties of graphene by capping with
p-conjugated organic semiconductor molecules. The off-state current is reduced while the on-state
current and mobility are either unaffected or increased. The semiconductors employed included
a-sexithiophene and hexadecafluorocopperphthallocyanine. Removal of the organic semiconductor
layer results in a return to the original electronic properties. This suggests that weak electronic
interactions with organic semiconductors can be a promising approach to favorably alter the transport
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properties of graphene. 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4737939]
Electronic devices consisting of a single layer graphene
have been of great scientific and technological interest due to
unique material and electronic properties,1,2 In particular,
very fast charge carrier transport combined with mechanical
and thermodynamic stability can be fundamental to promis-
ing nano-scale and sub-millimeter wave electronic devices
applications.2,3 However, since a single layer graphene has
no band gap in principle and the current in graphene field-
effect transistors (FETs) is difficult to turn off completely by
gate bias compared to silicon-based devices, this limits the
possible use of such transistors in digital circuits.4–6 There
have been previous attempts to improve the on-off current
ratio in graphene FETs.5–8 The electronic properties of gra-
phene based FETs can be tuned based upon electrical struc-
tures and interaction at interfaces, which influences device
performance.
1000 ꢀC under a 500 sccm hydrogen environment. The hydro-
gen gas was purged away at the end of the annealing process
and then ultra-high purity (99.99%) methane was circulated at
10 sccm for 5 min during the growing process. After growth,
the chamber was naturally cooled down below 180 ꢀC before
removal of samples.
In order to transfer a single layer graphene from the cop-
per film to prepared silicon dioxide-silicon substrate, the
conventional wet-transfer was processed.13 Graphene on
copper film was spin-coated with poly(methyl methacrylate)
(PMMA) at 4000 rpm for 1 min. The sample was kept in a
dessicator overnight for removing the solvent in PMMA and
then placed into dilute ammonium persulfate solution (0.5M)
to etch away the copper film. The PMMA-graphene film was
rinsed with deionized (DI) water several times to remove
Research and manufacturing of organic semiconductors
have been developing rapidly during the past few decades.9
Most of the studies on graphene in organic devices have
focused on its metallic conductivity for use as an
electrode.10–12 It must be noted that there have been no stud-
ies on FETs with hybrid graphene/organic semiconductor
active layers. It is well known that device characteristics
such as conductance, field-effect mobility, and on-off current
ratio in graphene FETs are reduced after the deposition of
most gate dielectrics on graphene when making dual-gated
devices.4 We report on the improvement in the electronic
properties of graphene by capping with p-conjugated organic
semiconductor molecules. We describe the electrical charac-
teristics of hybrid graphene/organic semiconductor FETs
employing a-sexithiophene (a-6T) and hexadecafluorocop-
perphthallocyanine (F16CuPc) for the organic materials.
Figures 1(a) and 1(b) show the schematic cross-section
and optical image of a hybrid graphene/organic semiconductor
FET. The device fabrication process starts with 1-10 X-cm
n-doped silicon substrate employed as the bottom-gate elec-
trode. It is thermally oxidized to result in a 290 nm thick sili-
con dioxide bottom gate insulator. A single layer graphene
was synthesized by a low-pressure chemical vapor deposition
(LPCVD) on a 500 nm thick copper film. The LPCVD process
is as follows. At first, the sample was annealed for 5 min at
FIG. 1. (a) Schematic cross-section and (b) optical image of a hybrid gra-
phene/organic semiconductor FET and (c) Raman spectra measured with a
442 nm blue laser. The inset shows AFM image of a single layer graphene
after graphene transfer.
a)Corresponding author. Email address: ananth.dodabalapur@engr.utexas.
edu.
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101, 033309-1
2012 American Institute of Physics
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033309-2
Ha, Akinwande, and Dodabalapur
Appl. Phys. Lett. 101, 033309 (2012)
residual solution. Prepared silicon dioxide–silicon substrate
was placed inside the DI water in an inclined manner and
was positioned under the PMMA-graphene film. DI water
was removed to allow PMMA-graphene to drift down onto
the substrate and then PMMA was removed with acetone
over 24 h.
0.03 and 0.02 cm2/V s in air, respectively. These values are
consistent with the highest reported mobilities for these
materials.14 In order to remove organic semiconductors from
hybrid graphene/organic semiconductor FETs, they were
subject to heat treatment using a substrate heater at
250–300 ꢀC under high vacuum. This causes the organic
semiconductor film to sublime away. Hybrid graphene/or-
ganic semiconductor FET possesses a channel width of 8 lm
and a channel length of 4 lm. Direct-current (DC) measure-
ments for the device characteristics were carried out using
Agilent 4155C semiconductor parameter analyzer.
Source/drain electrodes were patterned by electron-
beam lithography and lift-off, and then a chrome/gold
(2.5 nm/50 nm) bi-layer was deposited by thermal evapora-
tion. The sample was kept in high vacuum for 2 days to mini-
mize impurity incorporation. After completing the
fabrication process of graphene FETs, Raman spectra of sin-
gle layer graphene were measured, as shown in Figure 1(c).
The full width at half maximum (FWHM) of the 2D-peak is
ꢁ28 cmꢂ1. The intensity ratio between 2D and G peaks is
3.5. The D peak intensity is small or negligible. Atomic force
microscope (AFM) images of the graphene show a polycrys-
talline film, as shown in Figure 1(c).
Figure 2(a) shows the transfer characteristics of a hybrid
graphene/organic semiconductor FET employing a-6T semi-
conductor at a drain-source voltage of 0.1 V during the
A 15 nm thick layer of organic semiconductor (either a-
6T or F16CuPc) was deposited by thermal sublimation at a
rate of 0.1 A/s under a pressure of 10ꢂ7 Torr. Substrate heat-
˚
ing at 180-200 ꢀC was applied to improve the quality of
semiconductor films. Optimized bottom gate bottom contact
a-6T and F16CuPc FETs possess field-effect mobilities of
FIG. 2. (a) The transfer characteristics of single layer graphene, graphene
by capping with a-6T and graphene FETs after removing a-6T at the drain-
source voltage of 0.1 V during the sweep of gate bias from ꢂ70 V to 70 V.
The inset shows the chemical structure of a-6T and (b) the total resistance
along with diffusive transport modeling fit.
FIG. 3. The transfer characteristics of single layer graphene by capping
with (a) a-6T and (b) silicon dioxide. (c) The normalized resistance in single
layer graphene and graphene FETs by capping with a-6T and silicon
dioxide.
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033309-3
Ha, Akinwande, and Dodabalapur
Appl. Phys. Lett. 101, 033309 (2012)
sweep of gate voltage from ꢂ70 V to 70 V. The transfer char-
acteristics of a graphene FET are not degraded by depositing
organic semiconductor onto the graphene layer. It can be
noted that the transfer characteristics of hybrid graphene/or-
ganic semiconductor FETs return to its initial state (i.e.,
before organic semiconductor deposition) after removing
a-6T semiconductor layer. The electronic properties of gra-
phene can be tuned favorably using the capping layer of or-
ganic semiconductor.
In order to extract key parameters, a diffusive transport
model based on total resistance of the graphene device was
used.15
L
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
RTOTAL ¼ RCONTACT
þ
;
2
Wel n20 þ n½VGꢃ ꢄ
FIG. 4. The total resistance in single layer graphene and graphene FETs by
capping with F16CuPc along with diffusive transport modeling fit. The inset
shows the chemical structure of F16CuPc.
where l is the field-effect mobility, n[VG*] is the value of the
carrier concentration induced by the gate bias away from the
Dirac point, and no is the density of carriers at the minimum
conductivity point. Figure 2(b) shows there is good agreement
between the experimental data and diffusive transport model.
Optimized graphene/organic semiconductor FETs with a-6T
possess field-effect mobility of 1289 cm2/V s in air, which is
slightly improved from 1176 cm2/V s in single layer graphene
FET without a decrease in the on-off current ratio. The
extracted no is 1012 cmꢂ2 and width-normalized contact resist-
ance is ꢁ1.9 kX lm. Furthermore, removal of the organic
semiconductor layer from graphene FETs results in a return to
the original electronic properties. The results suggest that
there are weak reversible electronic interactions between a-6T
and graphene layers.
In summary, we have investigated hybrid graphene/or-
ganic semiconductor FETs with a-6T and F16CuPc. The
electronic properties of graphene based FETs can be favor-
ably tuned by capping with p-conjugated organic semicon-
ductor molecules, particularly F16CuPc. The off-state current
was reduced while the on-state current and mobility were ei-
ther unaffected or increased. This results from weak reversi-
ble electronic interactions between organic semiconductors
and graphene.
This work is supported by NSF-ECCS under grant
1028184 and an ONR Grant in the program of Dr. Chagaan
Baatar.
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